Isolation and characterization of spontaneously occurring TOL plasmid mutants of Pseudomonas putida HS1

Abstract

A strain of Pseudomonas (P. putida HS1) was found to resemble P. putida (arvilla) mt-2 in its ability to degrade toluene, m- and p-xylene, 1,2,4-trimethylbenzene (pseudocumene), and 3-ethyltoluene via oxidation of a methyl substituent and reactions of the meta-fission pathway. The ability to degrade these substrates by P. putida HS1 (PpC1) was shown to be encoded by a TOL (pDK1) plasmid as evidenced by: (i) spontaneous loss of the TOL-related phenotype after growth with benzoate, (ii) transfer of the TOL character from the wild type into cured recipients by conjugation, and (iii) isolation of a plasmid of identical molecular weight (120 X 10(6)) from both the wild type and an exconjugant obtained by mating wild type with a putative cured recipient. In addition to the isolation of apparent cured strains having lost the entire TOL-related phenotype, two additional mutant classes were observed after growth on benzoate. One class, represented by PpCT1, was unable to utilize the alkyl-substituted aromatic compounds but retained the ability to grow with toluene and benzyl alcohol. Analysis of PpCT1 revealed that it was unable to synthesize the TOL-encoded toluate oxidase and enzymes of the meta pathway but retained the ability to elaborate activities for toluene hydroxylase, benzyl alcohol, and benzaldehyde dehydrogenase, thereby mediating initial oxidation of toluene to benzoate, which was then further metabolized via enzymes of the chromosomally encoded ortho-fission pathway. A second class of mutants had lost the ability to utilize the hydrocarbons but could still grow with m-toluate but not p-toluate, 3,4-dimethylbenzoate, or 3-ethylbenzoate, intermediates in the oxidation of the corresponding hydrocarbons. Our such mutant, PpCM1, could no longer synthesize enzymes required for initial oxidation of the hydrocarbons, but was able to produce the toluate oxidase and enzymes of the meta pathway, thereby facilitating degradation of m-toluate. Neither PpCT1, PpCM1, nor a putative cured strain, PpCC1, reverted at detectable frequencies (less than 10(-9). Analysis of each strain for plasmid deoxyribonucleic acid revealed the presence of a single plasmid in each strain with the following molecular weights: PpCM1, 100 X 10(6) (pDKM1); PpCT1, 80 X 10(6) (pDKT1); PpCC1 20 X 10(6) (pDKC1). The results suggest that the TOL (pDK1) plasmid has undergone deletions giving rise to smaller replicons which either encode for only a fraction of the wild-type catabolic functions (pDKM1, pDKT1) or have lost all catabolic activities (pDKC1).